Electrical contacts for an electro-optic modulator

ABSTRACT

A contact structure and method of formation of a contact structure for an electro-optic modulator. Linear electrodes on a modulator crystal are covered with a dielectric layer. The electrodes are contacted by way of one or more vias through the dielectric layer. Contact pads are formed over the vias so as to contact the electrode and extend over several adjacent electrodes, providing significantly greater contact area for a driver chip applied to the modulator crystal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Rule 60 Division of U.S. application Ser. No. 525,280, filedMay 17, 1990 now U.S. Pat. No. 5,144,472.

BACKGROUND OF THE INVENTION

The present invention is related to the field of electrical connections.In particular, one embodiment of the present invention provides animproved method and apparatus for making electrical contacts to anelectro-optic modulator.

Optical modulators are well known to those of skill in the art and arecommonly used to modulate light in a variety of applications including,for example, laser scanning for non-impact printing, transmission ofdata in optical fibers, screen displays, film exposure, optical readingequipment, optical fourier transform generators, and the like. Manyoptical modulators operate by passing a light beam, usually amonochromatic coherent beam of light, into or through a crystal. Anelectrical field is imposed on the crystal via, for example, a set ofelectrodes on a surface of the crystal. The electrical field variesoptical properties of the crystal such its index of refraction. As theindex of refraction of the crystal varies, the crystal modulates thelight in a desired manner through variation of the electrical field.

One such optical modulator is described in greater detail in, Sprague etal., "Linear Total Internal Reflection Spatial Light Modulator For LaserPrinting," SPIE Vol. 299 (1982) which is incorporated by referenceherein for all purposes. Related modulators are described in U.S. Pat.Nos. 4,391,490, and 4,718,752, which are also incorporated by referenceherein for all purposes.

While prior art crystals have met with significant success, substantialproblems still remain. For example, the electrical field in themodulator disclosed in Sprague et al. is imposed by electrodes on adriver crystal which are mechanically engaged to the modulator crystal.This arrangement provides for poor contact to the modulator crystal and,therefore, poor response characteristics.

As an alternative to direct mechanical engagement of linear electrodeson the driver chip to the modulator crystal, linear electrodes could beformed on both the driver chip and the modulator crystal andsubsequently pressed together. Long electrodes would provide a longinteraction region for light and ease alignment problems. The use ofmatching linear electrodes on both the driver chip and modulator crystalalso presents difficulties however. For example, since a large number(2-50,000 and commonly 500-10,000) of the control electrodes arerequired on the modulator crystal and since the electrodes are ofnecessity very small (on the order of 1 to 10 microns wide) andseparated by a very small distance (also on the order of 1 to 10microns) it is exceedingly difficult to properly align the integratedcircuit driver chip electrodes to the crystal electrodes and often theelectrodes of the driver chip would be crossed with the electrodes ofthe underlying crystal, resulting in unsatisfactory device performance.

Additionally, any asperites from contamination or localizednon-uniformity in substrate processing when mechanically compressed formpressure-induced variations in optical field properties which conflictwith the uniform operation of the modulation device. Further, the smallsize of the electrodes renders them easily damaged, destroyed, orelectrically shorted to other electrical contacts in the process offorming the compression fitting. Still further, the linear arrangementof the electrodes results in either a very long, narrow driver chip(which becomes difficult to produce and, therefore, excessivelyexpensive) or the need to apply multiple driver chips (whichdramatically increases the magnitude of the alignment problems). Stillfurther, matching linear electrodes could not practically be repaireddue to the damage incurred in unseating and re-seating the smallelectrodes. Still further, the matching linear electrode arrangementwould result in the need for a cantilevered driver chip design in orderto provide an interface to the outside world. The cantileveredarrangement would often result in chip breakage, and the like. Ofcourse, even when such linear electrodes are properly aligned, thecoefficient of thermal expansion of the silicon driver chip and thecrystal differ by a significant amount and when heated the electricalcontacts may be displaced sufficiently to destroy electrical contacts,or short to neighboring electrodes, or smear the metal electrodes oneither or both the driver chip and the crystal.

From the above it is seen that an improved electrical contact for anoptical modulator and method of forming electrical contacts on anoptical modulator are desired.

SUMMARY OF THE INVENTION

An improved electrical contact for an optical modulator and method forforming contacts on an electro-optical modulator are disclosed. Themodulator includes a crystal onto which a plurality of electrodes areformed. In many modulators the electrodes will take the form of metalstrips on the surface of the crystal and the metal strips will have avery high aspect ratio. A dielectric layer is formed on the electrodesurface and provided with vias or apertures extending to a surface ofthe dielectric layer. The via for each electrode is offset from the viaof the next electrode to provide a separation between adjacent vias.Contact metal is formed over the contact via and extends over adjacentelectrodes, providing greater surface area for contact with theelectrodes from a driver chip. Optionally, solder bumps and barriermetal layers are also provided.

Accordingly, in one embodiment the invention provides a modulator forvarying at least one characteristic of light passing through themodulator. The modulator includes a crystal, the crystal responsive toapplication of an electric field thereon so as to change at least oneoptical property of the crystal; a plurality of high aspect ratioelectrodes on the crystal, the electrodes having a first spacingtherebetween; a dielectric layer over the plurality of electrodes, thedielectric layer comprising: i) an upper surface; and ii) at least oneaperture extending from the surface to each of the plurality ofelectrodes. The modulator further includes one or more conductive metallayers extending through each of the apertures and across the uppersurface over at least one adjacent electrode, forming a pad. The pad maybe, for example, circular. The conductive metal regions are contactedby, for example, linear electrodes on a driver chip through mechanicalengagement, attachment by way of soldering, asymmetrical contact mats,or the like.

A method of forming electrical contacts for an optical modulator crystalis also disclosed and includes the steps of, on the modulator crystal,forming a plurality of high aspect ratio electrodes on a surfacethereof; forming a dielectric layer on the surface; forming at least oneaperture in the dielectric layer for each of the electrodes, theapertures having a width less than about a width of the electrodes wherethe apertures contact the electrodes; forming a metal layer on thedielectric layer, the metal layer extending through the apertures tocontact the electrodes; etching the metal layer to form a bonding padassociated with each of the apertures, the bonding pads extending alonga surface of the dielectric layer over at least one adjacent electrode.

The invention herein provides a variety of benefits including greatlysimplified alignment. A good measure of the difficulty which will beencountered in aligning electrodes of two chips is provided by way ofthe "critical angle" and "critical distance." Critical angle is definedherein as the angle at which two chips may be canted from perfectalignment before a short between two electrodes occurs. Criticaldistance is defined herein as the distance the two chips may belaterally displaced from ideal alignment before a short occurs. Throughuse of the invention herein the critical angle is increased according toone embodiment from about 0.6 degrees to about 4.7 degrees. According tothe same embodiment, the critical distance is increased from about 5 μmto 35 μm. Obviously these numbers will be application dependent; theparticular example provided herein assumes 10 μm×0.5 mm lines and 2376lines with 7 pads per row. These differences will significantly impactthe commercial manufacturability of, for example, electro-opticmodulators and moves alignment problems from the realm of very difficultto the realm of readily achievable.

A greater understanding of the invention may be had by way of referenceto the description below along with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a modulator crystal;

FIG. 2a is a top view of a modulator crystal provided with a contactstructure according to one embodiment of the invention.

FIG. 2b illustrates the upper left hand portion of the crystal ingreater detail;

FIG. 3 is an enlarged cross-sectional side view of a portion of anelectro-optic modulator with a contact structure;

FIGS. 4a and 4b are more detailed top and cross-sectional side views ofa contact pad according to an alternative embodiment of the invention;

FIG. 5 schematically illustrates one arrangement of the contact pads intop view;

FIGS. 6a and 6b are side and top views respectively of a modulator boundto a driver chip;

FIGS. 7a to 7g illustrate formation of a contact structure according toone embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION CONTENTS

I. Description of the Contact Structure

II. Description of a Method of Forming the Contact Structure

I. Description of the Contact Structure

FIG. 1 is a top view of a conventional electro-optic modulator 2 of thetype well known to those of skill in the art. The modulator includes acrystal body 4 which may be, for example, lithium niobate (LiNbO₃),lithium tantalate (LiTaO₃), BSN, KDP, KD_(x) P, Ba₂ NaNbO₁₅, PZLT, orother materials which are responsive to electrical fields so as tomodify their electrical properties. The top of the crystal is providedwith a large number of substantially linear electrodes 6 which may be,for example, aluminum, chromium, titanium, molybdenum, gold, or thelike. In an alternative embodiment the electrodes comprise a dual ormultiple layer of metal, the first layer being, e.g., chromium foradhesion to the crystal and to act as a barrier metal, and a secondlayer of, for example, aluminum, to serve as a contact metal. A separatemetal layer may be provided to act as a barrier metal. Cap metals mayalso be provided in some embodiments.

Each of the linear electrodes 6 are parallel over all or a substantialportion of their length and have a high aspect ratio (e.g., of the orderof about 10:1 to 10000:1 and most commonly and between about 50:1 and150:1). For example, the electrodes 6 may have a width of about 0.1microns to 10 microns, a length of about 0.1 millimeters to 10millimeters. The electrodes are separated by a distance of about 0.1microns to 100 microns. In preferred embodiments the electrodes have awidth of about 5 microns, a length of about 0.5 millimeters and areseparated by a distance of about 5 microns (i.e., the lines have acenter to center distance of about 10 microns).

Most modulators have a large number of electrodes for precise control ofa light beam passing through the crystal. For example, in someembodiments the crystal is provided with about 100, 1000, 2000, 3000,4000, 5000, 10,000 or more such electrodes. According to a preferredembodiment the modulator is provided with a set of 4736 electrodes.

In operation, collimated, monochromatic light (indicated by hv) entersthe crystal 4 and passes through the crystal in the vicinity of theelectrodes. Voltages are selectively applied to electrodes 6 under thecontrol of a driver chip or integrated circuit (which is not shown inFIG. 1). As a voltage is applied to the electrodes the index ofrefraction of the crystal is modified in the region of the electrode,thereby altering a phase front of the light. The modulated light beamexiting the modulator may be used in a wide variety of applicationsincluding, for example, laser printers, video displays, datatransmission, image transmission, or the like. Operation of oneelectro-optic modulator is described in detail in Sprague et al.,previously incorporated by reference.

FIG. 2a is a top view of a modulator 2 according to one embodiment ofthe invention after fabrication of a contact structure thereon. FIG. 2billustrates the upper left hand corner of the modulator in greaterdetail For each electrode stripe on the crystal, a corresponding contactpad 8 is provided. According to a preferred embodiment the pads 8 arearranged in a staggered fashion such that the bonding pad 8b (which ispreferably circular) associated with the second electrode is displacedslightly to the right and downwards from the bonding pad 8a associatedwith the first electrode, the bonding pad 8c for the third electrode isdisplaced slightly to the right and downwards from the bonding pad 8bfor the second electrode, and so on. According to preferred embodiments,the pads lie along lines displaced by an angle (illustrated by theta) ofabout 1 to 30 degrees from lines formed by the electrodes with apreferred range of about 5 to 15 degrees.

By arranging the contact pads in as shown in FIG. 2, pads may beutilized which have a significantly greater diameter than the underlyingelectrode line width. Among other advantages, this greatly simplifiesalignment of driver chips. For example, a crystal provided with 5 micronelectrode lines on 10 micron centers may readily be provided withbonding pads which have a diameter of 35 microns and 70 micron center tocenter spacing. In preferred embodiments the diameter of the bondingpads (as viewed from above) is about 2 to 20 times the width of theelectrodes, with a range of about to 5 to 8 preferred.

According to a preferred embodiment the bonding pads for periodicelectrodes are provided in the same row (where a row is taken to hereinto be a line perpendicular to the electrodes and a column is taken to bea line parallel to the electrodes). For example, in the embodiment shownin FIGS. 2a and 2b, the bonding pads for the 1st, 5th, and 9th . . .electrodes are in the first row; the 2nd, 6th, and 10th electrodes areprovided in the second row, etc. In general, the bonding pad for everyn*i'th electrode is provided in the same row (where n is an integer andi is a counter representing an electrode number), the bonding pad forevery n*i+1'th electrode is provided in the same row, etc. By staggeringthe bonding pads in the manner shown, the contact region may occupy onlya small portion of the crystal area while providing greater contactintegrity, i.e. this arrangement provides maximum packing density withmaximum nearest neighbor distance.

According to one preferred embodiment, the spacing between bonding padsis approximately equal to the diameter of the bonding pads. Thepreferred bonding pad diameter is then determined from the equation:##EQU1## where: x is the binding pad diameter/spacing between pads;

A is the area available for the binding pads; and

C is the number of binding pads.

According to further alternative embodiments, the contact pads fordifferent regions of the crystal are displaced on the crystalpermitting, for example, the use of separate driver chips for differentregions of the crystal. For example, in the embodiment shown in FIG. 2,the bonding pads for the electrodes in the left half of the crystal areplaced in the upper left hand portion of the crystal while the bondingpads for the right half of the crystal are placed in the lower righthand portion of the crystal. The use of multiple driver chips permits,for fabrication of chips with greatly reduced area and reduced aspectratios which, as a general rule, will cost significantly less tofabricate. At the same time, alignment difficulties are substantiallydecrease and repair of the device becomes readily possible since thedriven chip can be replaced without destroying the modulator contacts.

FIG. 3 illustrates a small portion of the modulator 2 in cross sectionthrough contact pads 8a and 8e according to a preferred embodiment.Electrodes 6 extend into and out of the plane of the figure on thesurface of the lithium niobate crystal 4. A dielectric layer 12 of, forexample, polyimide, silicon nitride (Si₃ N₄), silicon dioxide (SiO₂), orthe like is provided above the electrodes. According to preferredembodiments the dielectric layer is formed through the application ofmultiple layers of dielectric material(s) for pin hole protection.

A pattern of apertures 14 is provided in the insulating layer. Thesidewalls of the insulation layer are preferably sloped as a naturalresult of the etching process. Metallization layer 16 is patterned todefine contact pads on the surface of the dielectric layer. The contactpads may be in any desired shape but are preferably circular orrectangular in shape and will generally be chosen to take advantage of apreferred etch geometry.

According to a preferred embodiment the contact metal 16 is formed intwo layers. The bottom layer is preferably a material which will providegood interlayer contact, act as a diffusion barrier to impurities, andadhere to the dielectric. Included among such materials are TiW, Ti, Cr,TiN, refractory metals, metal silicides, heavily doped polysilicon, andthe like. The second layer provides good crystal to chip contact and/orwetting metallization to solder. Included among such metals are Ni, Cu,Au, Ag, Sn, In, Pb, solder, and the like. The first layer is preferablyabout 75 to 1000 Å thick, and the second layer is about 1000 Å to 1 μmthick.

Solder bumps 18 are provided on metallization layer 16 for each of thecontact pads 8. Solder bumps 18 may be, for example, deposited PbSn,electroless plated SnIn, In, Sn, or the like. Solder bumps 18 arepreferably about the same height as diameter.

It will of course be recognized that while the invention is illustratedwith regard to solder bumps on the crystal, the solder bumps couldequally well be provided on the driver chip at selected locations.Alternatively, solder could be eliminated and conventional pressurecontact could be utilized. Still further, anisotropic conductive padscould be beneficially utilized in some embodiments. For example, RaychemCorporation of Menlo Park, Calif. has several products which conductperpendicular to the film plane which may be used in some embodiments.

FIGS. 4a and 4b illustrate the structure shown in FIG. 3 according to analternative embodiment in top view and cross section respectively.According to the embodiment shown in FIG. 4 first level vias orapertures 20 are provided through a first level dielectric 22. For 5micron electrodes, the first level vias may be, for example, 3 micron by8 micron rectangles. In general, the width of the vias will be slightlyless than (e.g., about 10% to 30% less than) the width of the electrodesso as to allow for photolithographic alignment errors. The length of thevias will be chosen by to provide good etching definition which in mostembodiments will mean that the length of the vias will be about 4 timesthe width of the vias. Multiple apertures 20 are provided for eachelectrode to provide good electrical contact and in a preferredembodiment 3 to 4 vias are provided for each electrode. The first leveldielectric is preferably polyimide or silicon dioxide, with polyimidepreferred for adhesion purposes.

Second level vias 24 are provided in a second level dielectric 26. Thesecond level vias are preferably substantially larger than the firstvias, but preferably do not extend over adjacent electrodes. Forexample, for the 5 micron electrodes shown in FIG. 4 the second levelvias are preferably about 10 by 30 micron rectangles. A single secondlevel via is provided for each electrode and encompasses all of theunderlying first level vias.

As with the embodiment shown in FIG. 3, bonding pad metallization 16 isprovided to fill the vias in the insulating layer(s) and solder bump 18is provided above the bonding pad metallization layer. As shown in FIGS.3, 4a and 4b, the bonding pad metallization and solder bumps mayencompass an area that extends well over adjacent electrodes. Forexample, when 5 micron electrodes are utilized the bonding padmetallization and solder bump may be about 35-40 microns in diameter.

FIG. 5 illustrates in top view an alternative preferred layout of theelectrode pads. The underlying electrodes are indicated by way of lines.This arrangement is generally in the form of a close packed hexagonalarrangement, although the lattice is slightly distorted because pads foradjacent electrodes are slightly displaced.

FIGS. 6a and 6b illustrate a modulator in a partially completed packagein side and top views respectively. The lithium niobate crystal 4 ispreferably mounted on a glass substrate 40. A driver chip 42 is bondedto solder bumps on the underlying crystal by way of heating the entiredevice so as to soften the solder and pressing the driver chip and thecrystal together.

Trace lines 44 on the surface of the dielectric lead to a tab orwirebond connection 46 which connects to a cable 48 of the type wellknown to those of skill in the art. Also shown in FIG. 7b for thepurposes of illustration is a set of pads to which a driver chip has notbeen bonded. A second driver chip would be bonded to the pads 50, wireboded to the trace lines, and the trace lines connected to aconnector/cable in a manner similar to the chip shown therein.

II. Description of a Method of Forming the Contact Structure

FIGS. 7a to 7g illustrate fabrication of the contact structure accordingto one aspect of the invention. In particular, FIG. 7a shows a crosssection of a lithium niobate crystal 4. A metal layer 52 having athickness of about 1 micron is deposited on the crystal by vacuumdeposition or similar process like. According to some embodiments, asecond metal layer is then applied, as shown in FIG. 4b. Thereafter, themetal is patterned by conventional lithography techniques so as to formelectrodes on the surf&W thereof, resulting in the structure shown inFIG. 7b. In preferred embodiments, the electrodes have a width of about5 microns.

As shown in FIG. 7c, a first layer of dielectric material 54 is formedon the surface of the device having a thickness of between about 1000 Åand 1 μm depending upon the dielectric which is used. Preferably, thedielectric is thicker than the underlying metal. The dielectric layermay be, for example, polyimide, SiO₂ or the like.

Thereafter, as shown in FIG. 7d, the first dielectric layer is patternedby conventional lithography techniques to form first level vias incontact with the electrodes. In preferred embodiments several vias (onlyone of which is shown in FIG. 7) are provided along the length of theelectrode. The first level vias preferably have a width slightly lessthan the width of the electrodes to allow for alignment tolerances.

Thereafter, as shown in FIG. 7e, a second level dielectric is depositedacross the surface of the device and patterned by conventionallithography techniques. In a preferred embodiment the second dielectriclayer is polyimide, although other materials could be utilized. Thesecond level dielectric preferably has thickness of between about 5000 Åand 2 μm with a thickness of about 1 μm preferred. The pattern of thesecond level dielectric is designed such that the apertures extend outto but not across the next underlying electrode in preferred embodimentsso as to ensure that pin holes in the first dielectric layer do notpermit contact of metal to adjacent electrodes.

Thereafter first and second metal interconnect layers are depositedacross the surface of the device and patterned simultaneously, againwith conventional lithography techniques. In preferred embodiments thefirst layer of metal 58 is TiW, Ti, Cr, TiN, refractory metals, metalsilicides, heavily doped polysilicon, or the like. The second layer 60provides good crystal to chip contact and/or wetting metallization tosolder. The metal pads formed preferably extend well over adjacentelectrodes so as to provide greater contact area. The pads arepreferably circular when solder is to be applied.

Thereafter, as shown in FIG. 7g a solder bead is formed on the metal padby conventional means such as vacuum deposition and etching orelectroless deposition. The solder bead may be formed of PbSn, SnIn, In,Sn, or any one of a variety of other metals. Electrode 62 on driver chip42 is attached to the solder bead by, for example, heating the entiremodulator crystal to a temperature of about 157° C. for In so as tosoften the solder, coarsely aligning the electrodes of the driver chipto those of the modulator, and pressing the driver electrode into thesolder so as to form an electrical connection. Preferably this step iscarried out in a reducing atmosphere. As seen from FIG. 7g, theelectrode of the driver chip need not be precisely aligned so as to makeeffective contact to the underlying modulator electrode. In fact, thedriver electrode could be displaced as far as the adjacent underlyingelectrode in some cases and still make effective electrical contact tothe desired electrode.

It is to be understood that the above description is intended to beillustrative and not restrictive. Many variations in the invention willbe apparent to those of skill in the art upon review of this disclosure.For example, while the invention has been illustrated primarily withregard to optical modulators, the invention is not so limited. Theinvention will also find use in other electrical connection schemeswherein a large number of electrical contacts must be made in a smallarea including, for example, acousto-optic modulators, and the like.Accordingly, the scope of the invention should be determined not withreference to the above description, but instead with reference to theappended claims, along with their full scope of equivalents.

What is claimed is:
 1. A method of forming electrical contacts for anoptical modulator crystal comprising the steps of:a) on said modulatorcrystal, forming a plurality of high aspect ratio electrodes on asurface thereof; b) forming a dielectric layer on said surface; c)forming at least one aperture in said dielectric layer for each of saidelectrodes, said apertures having a width less than about a width ofsaid electrodes where said apertures contact said electrodes; d) forminga metal layer on said dielectric layer, said metal layer extendingthrough said apertures to contact said electrodes; and e) etching saidmetal layer to form a bonding pad associated with each of saidapertures, said bonding pads extending along a surface of saiddielectric layer over at least one adjacent electrode.
 2. The method asrecited in claim 1 wherein the step of forming a plurality of highaspect ratio electrodes is a step of depositing metal on a surface ofsaid modulator crystal and etching selected portions of said metal. 3.The method as recited in claim 1 wherein the step of forming a pluralityof high aspect ratio electrodes comprises the steps of:a) depositing afirst chromium metal layer; and b) depositing a second metal layer, saidsecond aluminum metal layer.
 4. The method as recited in claim 1 whereinthe step of forming a metal layer further comprised the steps of:a)depositing a metal selected from the group of TiLu, Ti, Cr, TiN,refractory metals, metal silicide, and combinations thereof; and b)depositing a metal selected from the group of Ni, Cu, Au, Ag, Sn, In,Pb, and solder.
 5. The method as recited in claim 1 wherein the steps offorming a dielectric layer and forming at least one aperture furthercomprise the steps of:a) depositing a first dielectric layer on saidcrystal; b) forming a first set of apertures contacting said electrodes,said first apertures having a width less than a width of saidelectrodes; c) forming a second dielectric layer on said firstdielectric layer; and d) forming a second set of apertures in saidsecond dielectric layer, said second set of apertures having widthgreater than said first set of apertures.
 6. The method as recited inclaim 5 wherein the second set of apertures the width of said second setof apertures extend over a region between an electrode in contact withsaid aperture and an adjacent electrode.
 7. The method as recited inclaim 5 wherein:a) the step of depositing a first dielectric layer is astep of depositing a material selected from the group polyimide and SiO₂; and b) the step of depositing a second dielectric layer is a step ofdepositing polyimide.
 8. The method as recited in claim 1 furthercomprising the step of forming solder beads on said bonding pads.
 9. Themethod as recited in claim 8 further comprising the steps of:a) heatingsaid crystal so as to soften said solder beads; and b) pressingelectrodes of a driver chip into said solder beads.
 10. The method asrecited in claim 8 wherein the step of forming solder beads is a step ofelectroless plating said bonding pads with a material selected from thegroup of SnIn, In and Sn.